Photoconductive material has been intensively researched in recent years, and applied to photoelectric sensing elements such as solar batteries and image sensors, as well as electrophotographic photoreceptors. Conventionally, an inorganic material has been chiefly used for these photoconductive materials. For instance, a photosensitive layer, the main ingredient of which is an inorganic light conductive material such as selenium, zinc oxide or cadmium sulfide, has been widely used in electrophotography.
However, such inorganic photoreceptors have insufficient photosensitivity, heat stability, water resistance and durability, which are required for electrophotographic photoreceptors. Since selenium, for instance, crystallizes by heat or touch by humans, its characteristics as a photoreceptor are easily deteriorated. An electrophotographic photoreceptor using cadmium sulfide is insufficient with regard to water resistance and durability, and zinc oxide is insufficient with regard to durability.
Since electrophotographic photoreceptors such as selenium and cadmium sulfide have toxicity, manufacturing and handling are largely restricted because of environmental problems, which have become serious in recent years.
Various organic photoconductive materials have therefore drawn attention, to overcome such defects of inorganic photoconductive materials, and are being actively researched for use as a photosensitive layer of an electrophotographic photoreceptor. For instance, Japanese Patented Publication No. 50-10496/1975 discloses an organic photoreceptor having a photosensitive layer comprising polyvinylcarbazole and trinitrofluorenone, but sensitivity and durability are still insufficient. Therefore, a functional separation type electrophotographic photoreceptor which allots a carrier generation function and a carrier transport function to different substances has been developed.
Since the material of such an electrophotographic photoreceptor can be selected from a wide range, it is easy to obtain arbitrary characteristics, and as a result, an organic photoreceptor with high sensitivity and high durability is possible.
Various organic compounds have been proposed as a carrier generation material and a carrier transport material for the functional separation type electrophotographic photoreceptor. Especially, the carrier generation material defines the basic characteristic of the photoreceptor. This carrier generation material employs photoconductive substances for practical use, including a polycyclic quinone compound such as dibromoanthanthron, a pyrylium compound and eutectic crystal complex of a pyrylium compound, squarilium compound, phthalocyanine compound and azo compound.
Titanylphthalocyanine having a specific crystal type is known as having excellent characteristics. Titanylphthalocyanine has many crystal types, and each crystal type shows quite different performance from others. Especially, the crystal type titanylphthalocyanine having the maximum peak is 27.2.degree..+-.0.2.degree. of the Bragg angle of 2.theta. in the Cu-K.alpha. X-ray diffraction spectrum thereof, has remarkably high efficiency of photoelectrons, and an electrophotographic photoreceptor using this titanylphthalocyanine as a carrier generation material is extremely useful for the design of a high-speed printer, high-speed digital copier or high-speed facsimile.
The inventor has found that the efficiency of photoelectrons fell off when a Y-type titanylphthalocyanine having a significant peak at 27.3.degree. and 9.6.degree. in X-ray diffraction spectrum with extremely high efficiency of photoelectrons was heated or dehydrated in dry nitrogen. When Y-type crystals were put in the environment of normal temperature and normal humidity, they reabsorbed water, and the efficiency of photoelectrons recovered. That is, Y-type crystals are water-absorbing crystals, and the water molecules promoted dissociation of holes and electrons from excitons generated by light. It was considered that this was one of the reasons for high sensitivity. (Y. Fujimaki: IS&T's 7th International Congress on Advances in Nonimpact Printing Technologies, Paper Summaries, 269 (1991)). When such material is used as a carrier generation material, sensitivity characteristics due to the environment, especially humidity variation, may change causing problems in practical use.
On the other hand, to form a photosensitive layer, the titanylphthalocyanine to be used is finely dispersed in the organic solvent, adding binder polymers if necessary, and using various dispersion equipment, and the obtained dispersion is coated on the conductive substrate. Since the crystal stability of the compound having multi-form crystals varies depending on environmental conditions, the crystal is influenced by the solvent and binder, and the condition changes often in the dispersion. Since the titanylphthalocyanine crystals used in the present invention have especially high efficiency of photoelectrons, minor changes in crystallizing greatly influence the photoreceptor characteristics. Therefore, it is important to control changes in dispersion and to obtain long term crystal stability in the photosensitive layer against environmental factors.